5 ms·
This is very nitpicky, but does adding extra data at the bottom of the image count as steganography? I've always thought of steganography as something that is v
by StavrosK 6y ago
This is very nitpicky, but does adding extra data at the bottom of the image count as steganography? I've always thought of steganography as something that is very hard to detect, like changing the LSB of each pixel to a part of your message.
- onion2k 6y agodoes adding extra data at the bottom of the image count as steganography "steganography" is simply the act of hiding messages, so I don't see why this wouldn't count. What term for hiding information outside of the bounds of the image would you propose instead?
- 082349872349872 6y agoI'd say communicating outside normal means would be "using a covert channel", and reserve "steganography" for such communication as has been whitened to resemble noise[1]. Obviously tastes may differ, so someone else might call the former "steganography" and distinguish the latter as "deniable steganography", or at least "near-noise steganography". (with a little information theory, one can even communicate reliably via sub-noise steganography) cryptography vs. steganography? [1] compare Bacon (1605): https://archive.org/details/advancementofl00baco/page/256/mode/2up?q=biliteral https://archive.org/details/advancementofl00baco/page/256/mo... "... a method of expressing and signifying one's mind to any distance by objects that are either visible or audible — provided only the objects are but capable of two differences ..."
- onion2k 6y agoI'd say communicating outside normal means would be "using a covert channel", and reserve "steganography" for such communication which has been whitened to resemble noise[1]. Ah. I think I see where the confusion is coming from. You're saying this isn't steganography because the information isn't hidden in the image data. It's there in the 'plaintext'. I thought this is steganography because the image is hidden in the file data by exploiting the fact that PNG renderers don't display it. I'm going to change my mind and say that your definition is actually better than mine. The information itself isn't hidden. You don't need a trick, or a key, or any special knowledge to view it. The fact that common PNG libraries don't display it isn't really enough to say that it's enciphered at all - you can't really say a message is definitely hidden if you assume something about the PNG library the attacker is using to view it. I don't know what the right term is, but it probably isn't steganography.
- 082349872349872 6y agosecurity per obscurity? (skotadigraphy?)
- HelloNurse 6y agoMore "security by hoping they don't look". And in fact, the suggested uses of overlong PNG chunks in the linked article aren't particularly secure ones.
- steganograaf 6y agoThis might be how the term steganography is mostly used these days, but I disagree that it's the definition of steganography. An early example of steganography is writing on the scalp of a person and concealing it with the regrowth of hair: https://en.wikipedia.org/wiki/Steganography#History https://en.wikipedia.org/wiki/Steganography#History . The message is clear-text, you just have to know where to look for it. Which is the same as in the blog post. Some other non-computer examples: https://www.commonlounge.com/discussion/4bc16dbc2c7145ff87ad0f0d5401a242 https://www.commonlounge.com/discussion/4bc16dbc2c7145ff87ad... At the risk of "appeal to authority", my field of study is cryptography, and we used steganography in this broad sense. tl;dr: I agree with the blog post in that it is steganography.
- HelloNurse 6y agoIf it includes a marginally hidden container with an obvious structure this sense of "steganography" seems formally correct (secrecy without a secret key) but overly broad in practice. Consider, for example, that you can apply "normal" steganography to the image data in the overlong PNG content (claiming, for example, that you just "hid" a porn picture at the bottom of a cute kitten while there's a secret message in the whole composite picture).
- 082349872349872 6y agoApplying that scenario to Kabbalah, you may have just come up with an explanation for the canonical status of the Song of Songs.
- hombre_fatal 6y agoIt's hard for me to see how splitting hairs gets you any farther from calling it steganography when you find a place in a binary format where you can write arbitrary bytes. The covert channel is the payload handed over in broad daylight. How the secret is encoded or not is just impl detail. There's obviously a spectrum of how hidden the message is. e.g. tacking bytes onto the end of the image blob vs encoding your message by inc/decrementing rgba values away from the original. The former is trivial to remove in this case (can just diff images with different messages or run the image through an optimizer), the latter is impossible to remove though you can ruin the encoded message, but surely they both check the high level definitions of steganography.
- 082349872349872 6y agoI agree on the spectrum, the distinction I'm trying to make is one that Bacon wrote about in the seventeenth: https://archive.org/details/advancementofl00baco/page/254/mode/2up?q=biliteral https://archive.org/details/advancementofl00baco/page/254/mo... > "For if a letter should come into the hands of such as have a power over the writer or receiver, though the cipher itself be trusty and impossible to decipher, it is still subject to examination and question, unless there be no room to suspect or examine it." I doubt many slaves' heads were shorn looking for messages (and if they had been, the eavesdropping would be, as in QE, obvious), but overlong images are begging to be flagged by a linter https://news.ycombinator.com/item?id=24341108 https://news.ycombinator.com/item?id=24341108 , and anyone who naively stuffs plaintext there should hope no one ever runs strings(1). Anyway, as mentioned above, I don't care if the general case is called steganography, I'm just curious what then the specific case (of writing that has a "cover story") would be called?
- yodelshady 6y agoWouldn't I call the latter "encryption"? Nothing stopping you encrypting a steganographic message. Denying message existence isn't going to be possible in a system that also compresses data and leaks file size - the latter rules out having null states (all RF communication?) or large blocks of zeros (most, if not all, storage?). You could do something to avoid that, by doing a LOT of padding with random bits - but again, that's back to encryption, isn't it? Except encryption is something you can apply in the real world, where constant broadcast is usually impractical.
- mc32 6y agoDoes that mean ADS is a form of steganography since ordinarily you will not see the alternate stream?
- HelloNurse 6y agoSteganography is usually meant to refer to covert, deniable information encoding, while extra data is too unsubtle to be hidden from an adversary. It's a simple and convenient trick to hide data from unsuspecting users who don't look for secrets, a weaker threat model. Actual image-based steganography schemes, as used e.g. for watermarks, usually alter pixels in a way that is approximately (and hopefully) undistinguishable from plausible noise.
- m463 6y agoIt's invalid to HIde a Message that way, yOu aren't perMitted to do it.
- ShaneMcGowan 6y agoSpent too long thinking "What the hell is a himom"
- kd5bjo 6y agoThe PNG format supports arbitrary data being included in private (nonstandardized) chunks, which will be ignored by decoders that don’t recognize them. It’s easier to extract and more reliably hidden, so why bother inserting the extra data as pixels? cf. https://en.wikipedia.org/wiki/Portable_Network_Graphics#"Chunks"_within_the_file https://en.wikipedia.org/wiki/Portable_Network_Graphics#"Chu...
- chrisseaton 6y ago> [you can use] private (nonstandardized) chunks ... so why bother inserting the extra data as pixels? Because these extra chunk are super-obvious? As soon as someone looks in your file they're going to wonder what you've added these for. They will likely also not survive re-encoding. The point of modern stenography is that you can't tell it's there, and it's just a part of the core image data, and that it's completely deniable. How do you deny your nonstandard chunks?
- Beldin 6y agoThe "completely deniable" part is not necessary for steganography. Otherwise, you dismiss just about any examples of steganography prior to steganography on computers. Which is fine, but as far as i know, not the commonly accepted definition.
- chrisseaton 6y agoThe question was > so why bother The answer is - because it's more deniable. You bother because more deniable is better.
- crpatino 6y agoThe threat model is that of an active attacker that is able to tamper the message,- either inserting his own malicious data in the extra chunk, or zeroing it out if you are using strong authentication,- but that does care of the quality of the cover message (the image itself). By encoding the secret message in the pixels themselves, you ensure that the attacker cannot significantly alter the secret message without degrading the quality of the image itself. One application is watermarks containing copyright notices. The idea is that the infringing party cannot render the watermark undetectable without also degrading the quality (and by proxy, the economic value) of the image they are trying to plagiarize. Of course, that goal cannot be achieved by simply storing the secret message in the least significant bit of the image. You need to have a model of what type of tampering the attacker is going to perform, and then design an encoding that will be resistant to such attack.
- deleted 6y ago[deleted]
- yodelshady 6y agoIf it's compressed (it is!) how would you know? You can't simply do width x height x depth.
- jansan 6y agoPNG's IDAT chunk, which contains actual image data, is simply all lines of data compressed with the deflate algorithm. Once you inflate (uncompress) it, you have all data and can compare if width and height match. This way it is quite easy to detect if additional data was added.